Field of the Invention
The invention relates to a rhenium-containing protective layer for protecting a component against corrosion and oxidation at high temperatures. The component, in particular a component of a gas turbine, is exposed to a flue gas or the like at a high temperature.
The invention relates in particular to a protective layer for a component that contains a nickel-base or cobalt-base superalloy.
Protective layers for metallic components that are intended to increase the resistance to corrosion and/or oxidation of the components are known from the prior art. Most of the protective layers are known under the collective name MCrAlY, where M represents at least one of the elements from the group containing iron, cobalt and nickel, and further essential constituents are chromium, aluminum and yttrium. The latter may also be completely or partially replaced by an equivalent element from scandium or the rare earths.
Typical coatings of this type are known from U.S. Pat. Nos. 4,005,989 and 4,034,142. Moreover, it is known from the latter patent that an additional silicon fraction can further improve the properties of protective layers of the type mentioned above.
Furthermore, Published, European Patent Application EP 0 194 392 A discloses numerous specific compositions for protective layers of the above type with the addition of further elements for various applications. The element rhenium added in amounts of up to 10% by weight is mentioned as well as many other elements that can be added if desired. However, on account of relatively unspecific, wide ranges for possible additions, none of the protective layers described is qualified for particular conditions, such as for example those that occur at rotor blades and guide vanes of gas turbines with high entry temperatures which have to be operated for prolonged periods.
Protective layers which contain rhenium are also known from U.S. Pat. No. 5,154,885, Published, European Patent Application EP 0 412 397 A (corresponding to U.S. Pat. Nos. 5,273,712, 5,154,885, and 5,268,238), German Patent DE 694 01 260 T2 (corresponding to U.S. Pat. No. 5,455,119) and International Patent Disclosure WO 91/02108 A1 (corresponding to U.S. Pat. No. 5,401,130). The disclosures that can be found in these documents as a whole are incorporated in its entirety in the present application.
Ways of applying a protective layer to a component which is to be subject to high thermal loads in a gas turbine are to be found in Published, European Patent Application EP 0 253 754 A1 (corresponding to U.S. Pat. No. 4,743,462).
Efforts to increase the entry temperatures both in stationary gas turbines and in aircraft engines are of considerable importance in the specialist field of gas turbines, since the entry temperatures are important variables in determining the thermodynamic efficiencies which can be achieved by gas turbines. The use of specially developed alloys as base materials for components which are to be subject to high thermal loads, such as guide vanes and rotor blades, and in particular the use of single-crystal superalloys, makes it possible to have entry temperatures of well above 1000° C. By now, the prior art allows entry temperatures of 950° C. and above in stationary gas turbines and 1100° C. and above in gas turbines of aircraft engines.
Examples of the construction of a turbine blade or vane with a single-crystal substrate, which for its part may be of complex structure, are to be found in International patent Disclosure WO 91/01433 A1 (corresponding to U.S. Pat. No. 5,106,266).
While the physical load-bearing capacity of the by now highly developed base materials for the highly loaded components are substantially problem-free with regard to possible further increases in the entry temperatures, to achieve a sufficient resistance to oxidation and corrosion it is necessary to have recourse to protective layers. In addition to the sufficient chemical resistance to a protective layer to the attacks which are to be expected from flue gases at temperatures of the order of magnitude of 1000° C., a protective layer must also have sufficiently good mechanical properties, not least with regard to the mechanical interaction between the protective layer and the base material. In particular, the protective layer must be sufficiently ductile to be able to follow any deformation of the base material without cracking, since this would create points of attack for oxidation and corrosion. In this context, the problem typically arises that an increase in the levels of elements such as aluminum and chromium, which are to be able to improve the resistance of a protective layer to oxidation and corrosion, leads to a deterioration in the ductility of the protective layer, so that there will be an expectation of mechanical failure, in particular of the formation of cracks, in the event of a mechanical load which customarily occurs in a gas turbine. Examples of the way in which the ductility of the protective layer is reduced by the elements chromium and aluminum are known from the prior art.
International Patent Disclosure WO 01/09403 A1 discloses a superalloy for a substrate, which likewise contains rhenium. The document describes that the intermetallic phases formed by rhenium reduce the long-term stability of the superalloy. The problem can be alleviated by the addition of ruthenium.